The invention relates to a wellhead blowout preventer (BOP), and more particularly to a production BOP capable of sealing against a polish rod when in place, and against the central bore when the polish rod is not present.
The rams of a BOP may operate in different ways in closing off a well, or multiple BOPs may be used in a wellhead stack to provide different functions. In a production well, polish rod rams seal around a polish rod extending through the wellhead. Blind rams seal against each other across the central bore when no polish rod is in place.
A typical prior art BOP for a production wellhead is disclosed in U.S. Pat. No. 5,765,813 to Lam et al., issued Jun. 16, 1998, owned by Stream-Flo Industries Ltd., the assignee of this patent application. This type of BOP is commonly used in connection with pumping wells. With such wells, a sucker rod string is reciprocated or rotated to drive a downhole pump, which lifts the produced fluid to surface through a tubing string. The BOP is equipped with polish rod rams which can be advanced horizontally to seal around the vertical polish rod of the rod string, to prevent the upward escape of fluid. Alternatively, if the rod string is out of the well, the inner or front ends of the rams can be pressed together to cause closure of the wellhead assembly fluid passageway. Alternatively, the front ends of the rams can be configured as blind rams such that they seal against each other across the central bore when the polish rod is not in place.
More particularly, the prior art BOP includes a cross shaped housing forming a central vertical bore and a pair of coaxial, horizontal ram bores intersecting the central bore from each side. The BOP is commonly positioned in the wellhead assembly between the tubing head and flow tee. In such an embodiment, the BOP central bore forms part of the wellhead assembly fluid passageway. Within the BOP, a pair of rams is positioned in the horizontal ram bores. Means, such as screw jacks or actuators (mechanical, electrical, pneumatic or hydraulic), are provided at the outer ends of the ram bores, for advancing or retracting the rams into or out of the central bore, in order to close or open the central well bore.
Each ram comprises a generally cylindrical body, although other shaped ram bodies are known (ex. oval or square in cross section). The ram body generally comprises a steel core, preferably having an outer full bore diameter portion and a reduced diameter inner portion. The ram core inner portion is covered with and bonded to a layer of an elastomeric material, typically a nitrile rubber.
The ram bores, typically cylindrical, extend into the central bore and the bore surfaces combine at their intersection to form sealing areas. When the rams move into the central bore, the rubber surfaces of their inner portions seal against the sealing surfaces.
The rubber coated inner or front face of each ram is formed to provide a semi circular, vertically directed groove. Thus, when the polish rod of the rod string is present in the central bore, the ram ends encircle and press against it, to form a seal of the central bore. When the polish rod is not in the central bore, the ram ends compress together to form a solid block. In both cases, the circumferential seals of the ram side surfaces, with the sealing areas and the end face seals, combine to close the central bore and contain pressurized fluids.
One major problem associated with production BOP's is that the pressure acting from below on the closed rams may extrude the side rubber upwardly so that the circumferential seal with the sealing areas is lost. Another problem is that the end rubber bonded to the vertical end faces of the ram cores may tear loose from the core when high pressure is exerted from below. In severe conditions, such as injecting chemicals to close off a well, the rubber degrades quickly, causing the seals to fail.
As described in U.S. Pat. No. 5,765,813, the side surface of each ram core inner portion is formed to provide ribs extending outwardly therefrom and extending longitudinally thereof. The ribs are located so as to underlie those portions of the elastomer layer which seal against the sealing areas. The elastomer layer is thinned where it passes over the ribs, to preserve the cylindrical shape of the ram's inner portion. The ribs function to reinforce the elastomer layer against upward extrusion. Comparative pressure tests have shown that a ribbed ram is able to contain several times the pressure that a non ribbed ram could contain, before the seal was lost.
In accordance with a preferred feature of U.S. Pat. No. 5,765,813, the vertical end face of the core inner portion is indented, for example with a plurality of spaced apart transverse grooves or dimples or a single cavity, so that the base of the elastomer layer projects into these indentations. As a result, increased face surface area is provided to bond with the elastomer. In addition, the elastomer external of the indentations must shear from the elastomer in the indentations, before failure occurs and the elastomer separates from the steel. In this way, the resistance to failure of the elastomer/steel bond at the ram end faces is improved. The device of U.S. Pat. No. 5,765,813 improves the ability of the BOP ram to seal against the high pressure from below without losing the side seal, and better prevent against the rubber being torn loose from the ram.
Canadian Patent Application No. 2,260,655, published Aug. 2, 2000, and owned by Stream Flo Industries Ltd., describes a ram type BOP for high temperature applications. The BOP incorporates a generally L shaped seal element formed of a graphite or asbestos seal material, which is sandwiched between a bottom L shaped steel retainer plate and a semi cylindrically shaped steel top retainer plate. This design of BOP ram is well suited for brittle seal materials such as graphite and asbestos, which are used when steaming of the well may occur.
The BOP devices of the above patents do not adequately address all applications involving harsh chemical conditions. Rubber type seal materials (elastomeric seals) are subject to being degraded by the harsh conditions of some wellheads, for example where chemicals are injected. For instance, in wells in which heavy wax deposits accumulate in the annulus, outlet lines, and anywhere that the production fluid flows, a chemical inhibitor is introduced into the production fluid as a means to break down the deposits. While the inhibitor is effective in clearing the lines, it has the major disadvantage of acting as a dissolving agent on the O ring seals, rubber ram inserts and any elastomeric materials found inside the wells, causing these seals to fail prematurely.
Thermoplastic materials having superior chemical resistance are available, but have different compressibility and elastic properties than elastomeric sealing materials such as nitrile rubbers. Thus, the prior art BOP devices are not generally amenable to simple substitution of thermoplastics for the rubber sealing components.
In U.S. Pat. No. 7,137,610, issued Nov. 22, 2006, naming inventor Tony M. Lam, and owned by Stream-Flo Industries Inc., there is described a production BOP, and BOP rams, which incorporate thermoplastic seals at the front of the rams to seal against the polish rod.
There is a need for a production BOP which can reliably seal against a polish rod, but which can also seal the central bore when the polish rod is not in place. This could also eliminate the need for multiple valves and BOPs in a wellhead stack.